Re: [PATCH 2/2] qom: Assert that objects being destroyed have no parent

[Alex Bennée]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 16/12/20 08:53, Marc-André Lureau wrote:
>> 
>> On the principle, I fully agree. But the risk is high to introduce 
>> regression if objects are manipulated in strange ways.
>> 
>> I remember I wanted object_unref() to automatically remove itself from 
>> the parent when the last ref is dropped. I think there were similar 
>> concerns.
>
> unref and unparent are two very different operations; the former means 
> *I* am done with this object, the latter means *QEMU* is done with this 
> object (even though there may be a few reference held, e.g. on the call 
> stack or by RCU).  Since object_unparent operates on global state, you 
> can even call object_unparent if you don't own yourself a reference to 
> the object and just got the pointer from the caller.
>
> While unref is a "mechanical" operation of dropping a reference and 
> possibly freeing the object, unparent is an active operation that 
> includes for example dropping reference cycles or in general detaching 
> from other places that are known to hold references to this object.

This all sounds like good material for a QOM object lifetime section of
docs/devel/qom.rst

>
> This is not a concept that is specific to QEMU, I think I read somewhere 
> that LibreOffice's UI library does something similar, calling it "dispose".
>
> Paolo


-- 
Alex Bennée